Heracross

Mega Heracross has the highest base Attack stat in Battle Spot Singles. Couple that with its exceptional ability in Skill Link, making all its multi-hit moves hit 5 times, and its access to moves that take advantage of this ability, and you can see why Mega Heracross is one of the best wallbreakers in the metagame. Its coverage moves, including Pin Missile, Close Combat, Rock Blast, Bullet Seed, and Earthquake, are perfect for taking down the most common physical walls in Battle Spot, namely Cresselia, Suicune, and Zapdos. Aside from Mega Heracross's offensive capabilities, its good bulk before and after Mega Evolving allows Mega Heracross to take a few hits, and its defensive typing means it resists common types, such as Ground and Fighting, giving it a good matchup against Pokemon like Mamoswine and Lucario. This Bug / Fighting typing also gives Mega Heracross a resistance to Dark, giving it an edge against Swagger + Foul Play users.

Despite all this, though, Mega Heracross still has trouble beating a number of common offensive Pokemon. Heracross's low Speed and even lower Speed when Mega Evolved means that Heracross is outsped by a great number of the offensive threats. On top of that, while its defensive typing gives Heracross some good resistances, this typing also leaves Heracross weak to some very common attacking types, such as Fire and Flying, which means immense pressure is put on it from Blaziken and Talonflame.